implement the logic to set tile priorities based on current matrix

cc/picture_layer_impl.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/picture_layer_impl.h"
 
+#include "base/time.h"
 #include "cc/layer_tree_host_impl.h"
 #include "cc/math_util.h"
 #include "cc/quad_sink.h"
 #include "cc/tile_draw_quad.h"
 
 namespace cc {
 
 PictureLayerImpl::PictureLayerImpl(int id) :
     LayerImpl(id),
-    tilings_(this) {
+    tilings_(this),
+    last_update_time_(0) {
 }
 
 PictureLayerImpl::~PictureLayerImpl() {
 }
 
 const char* PictureLayerImpl::layerTypeAsString() const {
   return "PictureLayer";
 }
 
 void PictureLayerImpl::appendQuads(QuadSink& quadSink,
@@ skipping 54 matching lines @@
 }
 
 void PictureLayerImpl::didUpdateTransforms() {
   tilings_.SetLayerBounds(bounds());
   // TODO(enne): Add more tilings during pinch zoom.
   if (!tilings_.num_tilings()) {
     gfx::Size tile_size = layerTreeHostImpl()->settings().defaultTileSize;
     tilings_.AddTiling(contentsScaleX(), tile_size);
     // TODO(enne): handle invalidations, create new tiles
   }
+
+  gfx::Transform  current_transform = screenSpaceTransform();
+  double current_time =
+      (base::TimeTicks::Now() - base::TimeTicks()).InSecondsF();
+  if (last_update_time_ != 0) {

[epennerAtGoogle, 2012/11/30 16:48:34]

Should we move this condition into UpdateTilePriorities? We can still calculate
the distance, just not the velocity.

[qinmin, 2012/11/30 19:24:13]

using timedelta 0 when this happens, so that we won't calculate velocity in that
case.

On 2012/11/30 16:48:34, epennerAtGoogle wrote:

+    tilings_.UpdateTilePriorities(layerTreeHostImpl()->deviceViewportSize(),
+                                  contentsScaleX(),

[danakj, 2012/11/30 18:39:00]

can you DCHECK that contentsScaleX() == contentsScaleY()?

[qinmin, 2012/11/30 19:24:13]

Done.

+                                  last_transform_,
+                                  current_transform,
+                                  current_time - last_update_time_);
+  }
+  last_transform_ = current_transform;
+  last_update_time_ = current_time;
 }
 
 scoped_refptr<Tile> PictureLayerImpl::CreateTile(PictureLayerTiling*,
                                                  gfx::Rect rect) {
   TileManager* tile_manager = layerTreeHostImpl()->tileManager();
 
   return make_scoped_refptr(new Tile(
       tile_manager,
       &pile_,
       rect.size(),
       GL_RGBA,
       rect));
 }
 
 void PictureLayerImpl::SyncFromActiveLayer(const PictureLayerImpl* other) {
   tilings_.CloneFrom(other->tilings_);
 }
 
 }  // namespace cc